Is using Dropbox an option? Somebody told me a year ago, "...gotta have a Dropbox account...." I opened one, but have never used it. I realize there is a way to post photos in Dropbox, but don't know how another party retrieves them. Does the receiving part have to hold a Dropbox account as well? If so, then the appeal for this choice is minimal.

I have established that Dropbox does work as an anonymous way of transfer of photos. The sender uses a Dropbox account to upload an album of photos (or individual ones). Dropbox offers a "Share Link" feature that creates a link, including an access token. The sender can C & P that link to a PM, and the receiver can use the link to pick up the photo album. The receiver does NOT need to have a Dropbox account.

It is a faceless anonymous transfer, not unlike a drug deal (so I am told). The seller and buyer only agree upon a common drop point -- seller leaves the goods at the agreed upon location, the buyer picks them up later, without buyer/seller ever knowing each other. Well, maybe a bad example, but I think you understand.

Yes, photo hosting sites will work too, such as Flickr, Picasa, Zenfolio. But, most likely none of these sites provide the anonymity as does Dropbox.